Middle-East Journal of Scientific Research 7 (2): 175-179, 2011
ISSN 1990-9233
© IDOSI Publications, 2011
Identifying Dry Years by Using the Dependable Rainfall Index and its
Effects on the Olive Crop in Roudbar Gilan, South Western of Caspian Sea
Bahman Ramezani Gourabi
Physical Geography, Islamic Azad University, Rasht branch,Gilan, Iran
Abstract: Drought is one of the most important natural disasters which is probable to occur in all regions with
completely different climates and in addition to causing death. It results in many economic losses and social
consequences. Such as Studying the effects and losses caused by drought which include limitation or
shortage of agricultural production and drinking water resources, decreased rainfall and increased
evapotranspiration , Limited plant growth ,especially those of dry-farming. Lower levels of surface and ground
waters and increased immigrations. Etc. in the country is statistical period (1988-2007) for six stations in Roudbar
town were used for statistical analysis and calculating humid and dry years. The dependable rainfall index (DRI)
was the main method used in this research. Results has shown that during the said statistical period and also
during the years 1996-1998 and 2007 more than half of the stations had faced drought , comparing the available
data with those of dry and humid years it was found that drought affected agricultural products (e.g.olive) in
a way that during the year 1996 drought. Olive groves of Roudbar suffered the greatest damages, Whereupon
about 70% of the crops were lost.
Key words: Dependable Rainfall
Drought
Annual Rainfall
INTRODUCTION
In the twentieth century drought was mostly studied
as a climatic negative risk [1] and is one of the natural
hazards which is observable in different climates, of
course, with its gradual destructive effects [2]. Prediction
of drought can enable us in preventing problems. Drought
indices, usually expressed in numbers are used for making
better decisions by planners and designers. Moreover, it
is considered as one of the normal climatic features whose
occurrence is unavoidable. However, there are many
disagreements between researchers and policy-makers
regarding its characteristics. Studies have shown that the
lack of an accurate and precise definition hinders reaching
agreement on the nature of drought, which in its turn,
would lead to doubts and not showing a suitable reaction
on the managers and policy-makers' part. Generally. With
its various periodicities happening throughout the world.
Drought affects countries with different economy levels
including both developed and developing countries.
Investigating the raining fluctuation is one of the
most important climatic issues in Iran because the
temporal and spatial distribution of rainfall in this country
Roudbar
Olive
Gilan
is unbalanced [3] and its harmful effects can be observed
in all economic. Social, political and security aspects. of
all the rainfall-related problems with regards to both in
density. Frequency and occurrence and the spatial extent
and the damages made by drought, it can be said that it is
of great significance or it could be defined as the
continuous and abnormal shortage of rainfall in a
particular time period. In this definition, the word
"continuous" implies the persistence of the state of
shortage. While the word "Abnormal" refers to the
deviation of the index from normal or mean conditions.
Palmer [4] names frequency, intensity, extent
(comprehensiveness) and continuity as the four spatialtemporal characteristics of droughts [5]. Drought, as a
creeping and destructive phenomenon, has always
caused numerous domages in Iran through its history.
One of the most significant existing documents about
drought in Iran is an inscription from Darius that can be
found in persepolis in which drought is mentioned as the
second difficulty of the country [5].
As a whole, drought is a climatic event which could
be seen as agricultural, hydrologic. Meteorological,
economic, social and historical [7] form Smith[8] whose
Corresponding Author: Bahman Ramezani Gourabi, Associate Professor in Physical Geography,
Islamic Azad University, Rasht branch, Gilan, Iran.
Mob: +989111310993, E-mail: [email protected] & [email protected].
175
Middle-East J. Sci. Res., 7 (2): 175-179, 2011
features depend on the duration and intensity of the
extent of the affected area. Generally, drought is a
phenomenon which occurs in every area or country, with
either arid or humid climate and in our country, it is not a
new or unknown phenomenon, In fact, Iran’s natural
conditions and its geographical location are so that we
have always witnessed droughts and it can be said that
some of the regions are often faced with the phenomenon.
Therefore, identifying dry (drought) and humid years (wet
year). Their prediction and a proper and comprehensive
planning would results in reducing the effects and losses
caused by drought on man and his surrounding
environment [9].
Drought is defined as the shortage of rainfall in a
long-term period in a way that it could lead to soil
moisture deficit and a decrease in the flowing waters level
which would ultimately cause disorders in human
activities, natural vegetation and wildlife [10].
This phenomenon occurs in all climatic zones. Hence,
it is considered different from the word “Aridity” since
“Aridity is the permenant characteristic of some climates.
But drought is a temporary feature of all climates[11].
In a study with the topic “Determining Drought
threshold and Calculating the Dependable Rainfall Index
in Oroumieh Lakes Catchment stations”. By using
different drought evaluation methods showed the
advantage of the DRI because it can help a proper and
logical understanding of the rainfall threshold, especially
in agricultural planning and water resources management.
In his research with the topic “A study of the Drought
phenomenon By Using the Normal Precipitation
percentage in Cental Parts of Gilan.” Ramezani [12]
studied the occurrence of drought in those parts and
obtained result sindicated that with consideration of the
normal precipitation percentage, coastal regions and
plains are in drought conditions, while piedmont and
mountainous regions are in wet-year conditions.
In another research with the topic “Identifying the
Drought By using Precipitation Data in Anzali wetland
Basin Ramezani [13] specified the existing conditions of
the wetland in term of the drought conditions. Research
results suggest that Anzali wetland in the costal regions
and plains is considered to be in drought conditions,
while in the piedmont region it is in normal conditions and
in the mountainous region, it is in wet-year conditions and
that this trend would be almost continued for the next lo
years [14].
The objective of this paper was to analyze drought
through the rainfall element by using the dependable
rainfall index at Roudbar stations so that it could be of
some help to the planners of the preparation of this vast
country towards achieving its regional applied climatic
goals [15].
MATERIAL AND METHODS
For this research, the mean annual rainfall data of six
stations, geographically located in the south of Gilan
province, including Manjil, Lowshan, Paroudbar, Gilvan,
Shah-eShahidan and Shahr-e-Bijar collected during a
twenty-years period since 1988 to 2007 by Gilan Regional
Water Organization along with crop production rate data
collected by Gilan Organization of Agriculture were
selected for this study. Specifications of the said stations
and their spatial domains are shown in Table 1.
In order to process the data and to control their
quality (reconstructing and completing in complete data)
and also to assess the accuracy and homogeneity of the
data, Excell software was used. To calculate the
Dependable Rainfall Index (DRI) and to determine
wet/dry-years, this index was used as follows:
DRI( N P1 × P2 × P3...PN ) ) * 0 / 8
The presented scale for classifying the intensity of
rainfall and also to determine the rainfall quality through
the DRI is as follows:
NP = DR P
D = P DR
W = P GM
GM
Where, DR is the dependable rainfall index, 0.8 is the
constant coefficient, P is the given year’s precipitation, N
is the number of the annual precipitations’ observations
(duration of the statistical period), GM is the geometric
mean, NP is the normal precipitation range, D is the
drought theres hold and W is the number of the wet-year
threshold, DRI is one of the hydroclimatic indices which
is used for estimating the minimum water requirement of
a region and water resources as well.
Table 1: Geographical coordinates of Stations at the studied Region
Station
Manjil
Lowshan
Paroudbar
Shahr-e-Bijar
Shah-e-Shahidan
Gilvan
176
Elevation
Latitude Longitude From sea Type of
(0)
(0)
Level(m) Station
36-45
36-37
36-36
37-00
36-52
36-46
49-23
49-31
49-43
49-38
49-46
49-07
232
300
495
140
1780
311
Lysimetric
Lysimetric
Lysimetric
Lysimetric
Lysimetric
Lysimetric
Statistical
Year
1988-2007
1988-2007
1988-2007
1988-2007
1988-2007
1988-2007
Middle-East J. Sci. Res., 7 (2): 175-179, 2011
RESULTS AND DISCUSSION
After calculating the DRI for each of the
stations in the studied region, humid/dry years
and the statistical characteristics of droughts
and years of the stations, which included the
frequency of occurrence, extent (comprehensiveness)
and duration
(continuity)
were determined
(Table 4).
According to Table 4, major temporal and spatial
characteristics of stations in the studied region in
accordance with the dependable rainfall index can be
explained as follows.
During the twenty-years statistical period,
Manjil station with 5 years of drought, 12 wet-years and
3 years of rainfall, Lowshan station with 4 years of
drought, 12 wet-years and 4 years of rainfall, Paroudbar
station with 7 years of drought, 10 wet-years and 3 years
of rainfall, Shah-e-Shahidan station with 3 years of
drought, 13 wet-years and 4 years of rainfall, Gilvan
station with 4 years of drought, 10 wet-years and 6 years
of rainfall and lastly, Shahr-e-Bijar with 1 year of drought,
10 wet-years and 9 years of rainfall (Table 5) were put in
the normal domain.
The most important results obtained from the
statistical analysis of the rainfall parameter by using the
DRI at Roudbar stations are given in Table 2.
With consideration of the region’s mean rainfall,
Shahr-e-Bijar and Shah-e-shahidan stations had the
highest rainfall level, thus, they were among the most
humid stations. On the other hand. Gilvan, Lowshan,
Manjil and Paroudbar had the lowest rainfall level.
With the values of standard deviation and
precipitation changes coefficient taken into account.
Paroudbar and Gilvan Stations were the least stable.
While Shahr-e-Bijar station was the most dependable in
terms of rainfall distribution.
By using the DRI, relevant values for each station
were calculated and given in Table 3 and by considering
them as base values, the level of rainfall for each
statistical year of the stations was classified. DRI
calculated values, besides dry-year and wet-year
thresholds, specified the normal range. This value is a
relatively acceptable value for rainfall which in long-term
could be relied on for planning.
Table 2: Statistical specifications of rainfall in the studied stations
Station
Manjil
Lowshan
Paroudbar
Shah-e-hahidan
Average
268.03
223.72
328.13
590.94
Shahr-e-Bijar
1241.54
Gilvan
189.74
Geometric Mean
259.70
212.88
305.94
562.73
1228.80
180.58
Rainfall Range
226.9
321.7
480
802
166
Minimum
159.3
82.1
146
184
901
90.4
Maximum
386.2
403.8
626
986
1607
394.8
122.2
168.9
181.1
37
28
14
304.4
Standard
Deviation
66.6
70.12
24
31
65.3
Coefficient
of Changes
34
Table 3: Calculated DRI values for each studied station
Station
Manjil
Lowshan
Paroudbar
Shah-e-Shahidan
Gilvan
DRI
207.76
170.30
244.75
450.18
144.47
Shahr-e-Bijar
Normal Domain
Between
Between
Between
Between
Between
Between
207.76 & 259.70
170.30 & 212.88
244.75 & 305.94
450.18 & 562.73
144.47 & 180.58
983.04 & 1228.80
Dry-year Threshold
<207.76
<170.30
<244.75
<450.18
<144.47
<983.04
Wet-year Threshold
>259.70
>212.88
>305.94
>562.73
>180.58
>1228.80
983.04
Table 4: DRI values and the frequency distribution of humid. Dry and normal years of the selected stations
Parameter Station
DRI
Number of wet-years
Number of Normal years
Number of Droughts
Manjil
207.76
12
3
5
Lowshan
170.30
12
4
4
Paroudbar
244.75
10
3
7
Shah-e-Shahidan
450.18
13
4
3
Gilvan
144.47
10
6
4
Shahr-e-Bijar
983.04
10
9
1
177
Middle-East J. Sci. Res., 7 (2): 175-179, 2011
Fig. 1: Comparison of the planted area and the production level during the studied years
The most frequent wet-year period was an eightyears period at Lowshan station during the 1988-1995
period. Also, two 6-years periods were recorded for
Paroudbar (2002-2006) and Shah-e-Shahidan (19911995) stations.
The most comprehensive wet-year periods for all
stations, the phenomenon we mostly witness, have
been observed in 1991, 1993, 1995 and 2002.
The most comprehensive dry-year period for the
stations was at first observed in 1996 at Manjil,
Lowshan, Paroudbar and Gilvan stations and then, at
Manjil, Paroudbar, Shahr-e-Bijar, Shah-e-Shahidan
and Gilvan stations in 2007.
Table 5: Mean annual production (tn) and olive planted areas (hr) in
Roudbar
Production level (tn) Condition Fertile Planted Area (hr) Statistical year
8012.6
Normal
4713.3
1987-1988
14192.7
Wet-year
4340.3
1988-1989
14180.6
Dry-year
4051.6
1989-1990
6954.2
Wet-year
3759
1990-1991
9697.8
Wet-year
3232.6
1991-1992
5790
Wet-year
2895
1992-1993
6897.5
Wet-year
2759
1993-1994
9828
Wet-year
2457
1994-1995
6679.8
Dry-year
2457
1995-1996
5341
Dry-year
2420
1996-1997
8283.5
2802.5
1997-1998
8022
Wet-year
Normal
1910
1998-1999
5000
Wet-year
1910
1999-2000
5921
Normal
1910
2000-2001
6520
Wet-year
1890
2001-2002
6660
Wet-year
1850
2002-2003
6475
Wet-year
1850
2003-2004
5920
Dry-year
1850
2004-2005
5400
Wet-year
1800
2005-2006
5115
Dry-year
1787
2006-2007
A comparison of droughts with the level of olive
production in Roudbar shows that during dry years, its
production has decreased. This finding indicates the
importance of applied Climatology research in agriculture,
thus, conducting continuous
climatology
and
topoclimatology studies in piedmont and mountainous
regions could be effective in predicting drought, how to
scientifically deal with it and to avoid greater damages
which would result in less economic crop production
(Figure 1).
Based on the spatial distribution of dry and humid
years, Paroudbar and Manjil Stations with 7 and 5
frequencies of occurrence. Respectively had the
highest number of droughts, while Shah-e-Shahidan
and Manjil stations with 13 and 12 times, respectively
had the highest frequency of wet-years in the region.
The greatest sequence of the drought period was
that of two consecutive 3-years periods at Lowshan
and Paroudbar stations during 1996-1998 and also
that of a two- years period at Manjil station during
1996-1997.
CONCLUSION
In order to determine and specify dry and humid
years at six stations in Roudbar town, the dependable
rainfall index (DRI) was used because the estimation of
the minimum water requirement of a region. Especially in
terms of agriculture, was a chievable through this index.
Furthermore, of the statistical means (arithmetic and
178
Middle-East J. Sci. Res., 7 (2): 175-179, 2011
geometric). Only the geometric mean was used. The said
method is usually used to determine mean values of the
phenomena which are changed at different times.
Statistical analyses at the said region’s stations showed
that Paroudbar and Manjil stations with 7 and 5 cases,
respectively, had the highest drought frequencies. Also,
the frequency of wet-years along with the most
consecutive and comprehensive wet/dry-years periods of
the stations were determined by the DRI and its
comparison with the olive production level. Moreover,
drought related years which were identified y the abovementioned index were consistent with agricultural
droughts and reduced production levels. This finding can
be used in predicting the trend, future planning and
preparing the region for producing olive and other
products.
6.
7.
8.
9.
10.
11.
12.
REFERENCES
1.
2.
4.
5.
Gonzalez, J. and M. Vieente, 2004. Drought pattern
and its ecological effects in east Spain during the
second half of 20th century, Mill press, Rotterdam.
Rajaei, A., 2003. Applying Natural Geography in
Urban and Rural Planning. Samt publication, pp: 3.
Palmer, W.C., 1965. Meteorology drought, research
paper office of climatology, 45, us, weather bureau,
pp: 58.
Zahedi, M. and R. Ghavidel, 2007. Determining the
Drought Treshold and Cakulating the Dependable
Rainfall Index for the Stations at Oroumieh Lake
Catchment, J. Geographical Researches, 59. Tehran
University, pp: 21-23.
13.
14.
15.
179
Farajzadeh, .M., D. Movahed and Ghaemi, 1995.
Drought in Iran, J. Agricultural Knowledge, 4: 31.
Comp, 1994. Global environmental issue , Rutledge ,
New York, pp: 56.
Smith, K., 1998. Environmental Hazards, Rutledge,
New York, pp: 79.
Kordovani P., 2001. Drought and the Ways to
Confront It in Iran, Tehran University publications,
pp: 5-23.
Ghayour, H. and A. Masoudian, 1997. Magnitude
and Extent of Droughts in Iran, J. Geographical
Researches, 45: 26.
Khosh Akhlagh F. Ghaemi and M. Zahehi, 2006. A
Study of Monthly Patterns of Dry-and wet-years in
Iran, J. Geographical Researches, 45: 37.
Ramezani, B., 2005. A study of the Drought
Phenomenon by using the Normal Precipitation
Percentage Index in the central Regions of Gilan
Province, Biyaban Scientific and Research J., 2:
Tehran University.
Ramezani. B., 2004. Identifying the Drought Trend by
Using Precipitation Data from the Anzali Wetland
Basin, Journal of the Geographical Space, No.12,
Ahar University.
Ramezani, B., 2008. Recognition of climatic drought in
south western of Caspian sea, Red book,IAHS pub,
pp: 319: 319.
Farajzadeh, M., 2007. Climatology Techniques.
SAMT publications.